The present invention resides in an electronic flash apparatus for photographic applications which includes a light meter fed from a DC operating voltage supply. The electronic control circuitry containing the light meter extinguishes the flash bulb used in conjunction with the flash apparatus as soon as the scene to be photographed has reflected a predetermined amount of light onto the light-sensitive element of the light meter.
Heretofore, an electronic flash apparatus has been available for the purpose of omitting the requirement to set the diaphragm of a photographic camera prior to each exposure because of the different distances between the flash apparatus and the scene to be photographed. The flash apparatus serves under these conditions the purpose of illuminating the scene to be photographed. With such electronic flash apparatus, the flash bulb becomes automatically extinguished when the scene to be photographed is sufficiently illuminated. In this manner, the object or scene being photographed is correctly illuminated in view of the condition that the duration of the flash is controlled through the amount of light reflected by the scene upon a light sensitive element.
The light meter includes, for this purpose, an integrating arrangement which integrates the electrical current derived from the light-sensitive element and actuates in response thereto, a control arrangement, as soon as the integrated current attains a predetermined value. The conventional means for integration is in the form of a capacitor which integrates the current derived from the light-sensitive element. The capacitor becomes charged, and when the voltage across the capacitor attains a predetermined value, the control arrangement becomes actuated and the flash bulb becomes thereby extinguished. It is known in the art that for the purpose of extinguishing the flash bulb, a discharge tube is used in parallel with the flash bulb. The discharge tube has a considerably lower internal resistance than the flash bulb when it is ignited. The discharge tube becomes thus ignited through the control arrangement or circuitry when the capacitor has attained a predetermined voltage level. The lower internal resistance of the ignited discharge tube causes the discharge of the storage capacitor which provides electrical energy for the flash bulb, and as a result the flash bulb becomes extinguished.
An electronic arrangement provided with such a control circuit, however, has the disadvantage that the photographic process or operation can be interfered with through the prevailing ambient light, as well as through the photographic flash resulting from another camera in the neighboring vicinity. The ambient light causes current to be generated through the light-sensitive element before actuation of the flash apparatus, and this current is applied to the capacitor which performs its integrating function. As a result, the capacitor possesses unpredictable initial voltage prior to initiation of the flash. In view of this condition, the time interval during which the predetermined voltage across the capacitor is attained, is no longer well defined. Furthermore, a remote flash from another apparatus in the vicinity, can actuate the control arrangement or circuitry of the flash apparatus under consideration, at an undesired instant of time. The result of such action is that the storage capacitor becomes thereby discharged.
In order to avoid interference through the ambient light, it has been previously proposed to omit the integrating capacitor and to provide a single and sole photoresistor of predetermined characteristics. With such an arrangement, however, interference resulting from remote flashes can still not be avoided.
It is also known in the art, to connect the light meter into the circuit first at the beginning of the flash through a switch or shutter mechanism. With such design, disturbances or interferences can be avoided from either the ambient light or remote flashes. Such design, however, requires rather careful and meticalous mechanical construction which is considerably complex and thereby costly to manufacture. At the same time, such mechanism and design does not operate reliably.
In accordance with the present invention, on the other hand, an arrangement is provided in which the ambient light as well as remote flashes are taken into account through a very simple and reliable manner. The present invention provides that the operating DC voltage is realized through the ignition of the flash bulb by an ignition current from an ignition circuit for the flash bulb. The arrangement in accordance with the present invention is based on electrical structure rather than mechanical parts, and thereby the control circuitry is principally switched on only during the flash. In this manner, interferences from remote flashes and ambient light cannot be incurred.
The flash bulb of an electronic flash apparatus becomes ignited through the application of an ignition capacitor operating in conjunction with an ignition transformer. The ignition capacitor is connected to the primary winding of the ignition transformer, and when a synchronizing .[.shutter.]. .Iadd.trigger .Iaddend.cable becomes switchingly actuated, the ignition capacitor discharges and thereby produces a high voltage ignition pulse in the secondary winding of the ignition transformer. This high voltage pulse is applied to the flash bulb.
In carrying out the concept of the present invention, the primary winding of the ignition transformer can form a closed series circuit with the ignition capacitor and with a charging capacitor, when igniting the flash bulb. The voltage across the charging capacitor results from .[.charging.]. .Iadd.discharging .Iaddend. of the ignition capacitor and serves as the DC operating voltage for the light meter.
When the electronic flash apparatus is switched on, one electrode of the ignition capacitor is connected to a voltage divider associated with the flash bulb. The other electrode of the ignition capacitor leads to ground potential, by way of the primary winding of the ignition transformer. The ignition capacitor becomes thus charged to a corresponding voltage.
When the synchronizing shutter cable is actuated so that it forms a closed circuit through it, the charging capacitor becomes switched to the ignition capacitor. One electrode of this charging capacitor is connected to ground potential. With this arrangement the primary winding of the ignition transformer forms a closed series circuit with the ignition capacitor and with the charging capacitor. The ignition capacitor, thereby, transfers a portion of its charge to the charging capacitor, by way of the primary winding of the ignition transformer. When, in accordance with the further provision of the present invention, the capacitance of the charging capacitor is large compared to that of hte ignition capacitor, the largest proportion of the charge of the ignition capacitor becomes transferred to the charging capacitor. As a result, the equalizing current flows from the charged ignition capacitor to the uncharged capacitor by way of the primary winding of the ignition transformer. Through this current flow through the ignition transformer, the flash bulb becomes ignited.
The voltage resulting across the charging capacitor through rapid charging of the latter to its maximum value, remains first constant near this maximum level, and forms the DC operating voltage for the light meter.
In a further embodiment of the present invention, the light meter can contain a series circuit to which the DC operating voltage is applied and which includes a light sensitive element in the form of a photoresistor, an adjustable charging resistor, and an integrating capacitor.
The adjustable charging resistor must be set in accordance with the sensitivity or speed of the film being used. This setting remains as the only setting to be carried out by the photographer. The setting can remain for as long as the same film material is used. After the flash bulb becomes ignited, the current produced by the photoresistor becomes integrated by the capacitor. This current through the photoresistor is a function of the brightness or illuminating intensity and the distance of the flash bulb from the scene or object to be illuminated. The integrating capacitor attains a predetermined voltage after a predetermined time interval suitable for illuminating the film.
In accordance with a further embodiment of the present invention, the voltage of the integrating capacitor can be applied, through a protective resistor, to the cathode-control electrode path of a controlled silicon diode or thyristor. This controlled silicon diode or silicon rectifier ignites a discharge tube when controlled so that it is in the conducting state. This discharge tube lies in parallel with the flash bulb, in the conventional manner. The internal resistance of the discharge tube, when ignited, has a considerably lower value than the internal resistance of a flash bulb.
Through the large current due to the ignition of the discharge tube by the thyristor, the storage capacitor which feeds the flash bulb becomes discharged at an earlier instant of time than if it were to be discharged only through the flash bulb itself. As a result, the flash bulb becomes extinguished at the correct instant of time determined by the illumination required for the specific film being used in the camera.
Another specific feature of the present invention resides in the manner in which the thyristor and the discharge tube are interconnected.